Cathode ray tube magnetic deflection circuit



June 2, 1970 J, L, MYERS 3,515,933

CATHODE RAY TUBE MAGNETIC DEFLECTION CIRCUIT Filed May 10, 1968 2 Sheets-Sheet 1 pmoce ART J'aHN L. M YEQS INVENTOR.

,0?- TOQA/Ey June 2, 1970 J. L. MYERS 3,515,933

CATHODE RAY TUBE MAGNETIC DEFLECTION CIRCUIT Filed May 10, 1968 2 Sheets-Sheet 2 1: 1 Q- 8 /o/m/ A. Ma /as United States Patent 3,515,933 CATHODE RAY TUBE MAGNETIC DEFLECTION CIRCUIT John L. Myers, Panorama City, Calif., assignor to Tasker Industries, Van Nuys, Calif., a corporation of California Filed May 10, 1968, Ser. No. 728,209 Int. Cl. H011 29/76 US. Cl. 315-27 9 Claims ABSTRACT OF THE DISCLOSURE The circuit improves high-frequency feedback by sensing the magnetic flux produced in the yoke due to inductive current independently of the capacitive component of the yoke current due to distributed capacitance. The flux sensor senses only rate of change of flux which is integrated to provide a measure of total flux. The total flux signal is used as a feedback signal to provide proportionality between signals having frequencies near and higher than the yoke resonant frequency, and the CRT heam deflection.

This invention relates to cathode ray tube magnetic deflection circuits and, more particularly, to such a circuit that provides increased bandwidth over those presently known and hence more faithful reproduction on the screen of the tube of an input signal having high frequency components.

In many present day applications, cathode ray tubes (CRT) are used to present visually information readout from a computer or other device that stores information in the form of electrical signals. Often, such electrical signals represent complex waveforms, such as square waves, triangular waves, or other waveforms that contain high frequency components that must be correctly reproduced by the CRT in order to present the correct waveform visually.

Because of the wide deflection angle obtainable electromagnetic deflection systems are generally employed, as opposed to electrostatic. In an electromagnetic deflection system, a yoke surrounds the rear end or neck of the CRT and the current flowing through the coil in the yoke controls the deflection of the electron beam emanating from the CRT electron gun. Generally, the yoke is driven from a power operational amplifier, which is provided with current feedback to maintain the desired linearity between input signal amplitude and beam deflection.

As is known, each magnetic deflection yoke has a particular resonant frequency. With input signals having frequencies below the yoke resonant frequency, presently used current feedback techniques are adequate to maintain proportionality between the input signal amplitude and the amount of electron beam deflection. However, when the input signals contain frequency components that are above the resonant frequency of the yoke, they are not faithfully reproduced. This is because, in the case of an input signal having a frequency above or near the yoke resonant frequency, the current through the yoke is divided into two significant components. One component is in phase with the deflection flux produced by the yoke, and the second component is of opposite phase. The second or capacitive, component subtracts from the first or inductive component and, consequently, the feedback current contains large errors. The present invention obviates the foregoing disadvantage.

The present invention improves high-frequency feedback by sensing the magnetic flux produced in the yoke due to inductive current independently of the capacitive component of the yoke current due to distributed capaci- 3,515,933 Patented June 2, 1970 tance. Most available and practical flux sensors do not sense total flux, but only the rate of change of flux. Hence, the rate of change of flux is detected and integrated to provide a measure of total flux. This signal representing total flux is used as the feedback signal to provide proportionality between signals having frequencies near and higher than the yoke resonant frequency, and the CRT beam deflection. In this manner, the effect of the aforementioned second component of yoke current is eliminated.

The invention, together with further features and advantages thereof, will be better understood from the fol lowing description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of a prior art deflection circuit;

FIG. 2 is a diagram of a basic circuit embodying the invention; and

FIGS. 3, 4, 5, 6, 7 and 8 are diagrams of different circuits embodying the invention.

Before describing the present invention in detail, it is believed to be in order to look at the requirements for yoke drive circuitry wherein the prior art circuitry does not meet all the requirements. The basic requirement for writing symbols of good quality on the face of a CRT is that the displacement d of the point of impingement of the electron beam from a reference point be related to the input signal E, by a linear proportionality factor k so that d=k E,

Assuming that distortions due to CRT geometry, etc. have been offset by a correction network or other means, the writing spot displacement is proportional to the electron beam displacement angle from the reference angle. For an electromagnetic deflection system, the deflection angle is proportional to the magnetic flux in the yoke, which is in turn proportional to the current I through the yoke. The desired proportionality is then obtained if where k; is a linear proportionality factor. Current feedback is utilized to effect the desired proportionality. In the present state of the art using available magnetic yokes, the shortest rise time is of the order of one microsecond in response to an input signal having virtually instantaneous rise time.

The principal reason for the foregoing limitation is that at input signal frequencies near and above the resonant frequency of the yoke, the yoke current I is divided into an inductive component I in phase with the flux in the winding and a capacitive component 1 of opposite phase. The capacitive component is due to current flow in the distributed capacitance of the yoke, yoke wiring and sometimes amplifier output circuitry. Thus, current flow in the yoke is As a consequence, the feedback current contains large errors. Thus far, the only way to eliminate such errors has been to severely limit the high frequency gain, which is obviously undesirable.

FIG. 1 illustrates a conventional prior art circuit for driving a magnetic deflection yoke. An input signal is applied to input means such as a terminal 10, and thence through an input amplifier 12 and a resistor 14 to a power operational amplifier 16. The output of the amplifier 16 drives a magnetic deflection yoke 18 for a CRT (not shown). The end of the yoke 18 not connected to the amplifier 16 is grounded through a resistor 20. A capacitor 22 represents the distributed capacitance of the yoke 18. Current feedback is provided by resistance means such as a resistor 24 connected between the input of the amplifier 16 and the end of the yoke 18 adjacent ground. The voltage drop across the resistor 20 provides negative feedback to the input of the amplifier 16. Since the feedback voltage is only the drop across the resistor 20, it represents total current flow through the yoke 18, which is set forth in Equation 3. Thus, the voltage drop across the resistor 20 is not truly repersentative of the deflection producing inductive current flow through the yoke 18.

A basic embodiment of the invention is shown in FIG. 2. An input deflection signal supplied to the terminal is supplied through the input amplifier 12 and resistor 14 to the operational amplifier 16, whose output drives the yoke 18. An inductive pickup coil 26 is wound around or located adjacent to the yoke 18, and detects the change in flux produced by the yoke. The current induced in the coil 26 causes a voltage drop across a feedback resistor 28, which is connected to an amplifier 30 and a capacitor 32 connected in parallel and thence through a resistor 34 to the input of the amplifier 16. The amplifier 30 and capacitor 32 act as an integrator to provide a feedback signal to the amplifier 16 representative of true inductive current through the yoke 18.

In FIG. 3, the inductive pickup coil 26 has been eliminated, and the feedback resistor 28 has been connected directly to the yoke 18. Since the inductive current through the yoke is directly proportional to the integral of the voltage across the yoke the current through feedback resistor 34 is proportional to deflection current. At high frequency, considerable current will flow through capacitor 22, but this current does not constitute a feedback error as it would in FIG. 1.

A major simplification of the circuitry is accomplished in the circuit illustrated in FIG. 4. There, capacitance means such as a capacitor 36 is placed in series between the amplifier 12 and the power operational amplifier 1 6. The resistor 24 in parallel with the amplifier 16 provides the feedback path. In that embodiment, the capacitor 36 acts as the analog of the yoke inductance, which, together with the resistor 24 in the feedback path, provides the necessary integration of the change in flux in the yoke 18. In effect, the capacitor 36 converts the power amplifier stage into a differentiator.

Thus far, consideration has been directed primarily to the high-frequency feedback loop. However, low-frequency feedback should also be employed to provide lowfrequency and direct current stability. Such an arrangement is shown in FIG. 5. The circuit there shown is quite similar to that shown in FIG. 4, except that a resistor 38 is interposed between the amplifier 16 and the yoke 18 and feedback resistors 40 and 42 are added. The resistor 40 is connected across the amplifier 16 and the resistor 42 is connected across the capacitor 36. Only the change in yoke current is fed back through the resistor 24 and integrated, because the resistor 24 is connected between the resistor 38 and the yoke 18 and the input to the amplifier 16. Conversely, the yoke current is detected and fed back by the resistors 40 and 42 to provide low-frequency and direct current stability, with the resistor 38 serving the same function as the resistor 20 shown in FIG. 1. Otherwise, the circuit operates in the same way as that shown in FIG. 4.

The embodiment of the invention shown in FIG. 6 includes two features not included in the embodiments heretofore described. It is known that certain distortions of the magnetic field in the cathode ray tube can occur due to the effect of yoke mounting and other second order effects. These effects can be at least partially compensated for by providing additional feedback, with the feedback path or paths being responsive to particular frequencies. In the present case, two additional feedback paths are provided. One comprises a capacitor 44 and a resistor 46 connected in series across the amplifier 16 and resistor 38, and the second comprises a capacitor 48 and resistor 50 similarly connected. As noted, these net- '4 works tend to balance out the effects of tube and yoke mountings, etc.

The second feature included in the embodiment of FIG. 6 is a signal limiter 52 connected between the input amplifier 12 and the capacitor 36. It is important in all embodiments that the integrator not saturate, because it would integrate errors during its saturation intervals. In the embodiments shown in FIGS. 2 and 3, this is a requirement of the integrator (30, 32). In the embodiments shown in FIGS. 4 and 5, it is a requirement of the loop that includes the resistor 24, the capacitor 36 and the power amplifier 16. In the embodiment of FIG. 6, the same action is provided by the limiter 52. The signal limiter 52 may be any well known device, such as a biased diode bridge or other arrangement, that will limit the amplitudes of input signals to a desired value.

The embodiment of the invention shown in FIG. 7 is very similar to that shown in FIG. 6 except that lowfrequency feed-back resistor 40 and the current sensing resistor 38 have been relocated. This permits more choice in selection of the frequency at which the high-frequency mode of operation takes over control from the low-fre quency mode. Another advantage of this arrangement is that it permits more of the low-frequency mode feedback to be derived from the current sense resistor 38 to the exclusion of the less stable yoke winding resistance (not shown).

The embodiment of the invention shown in FIG. 8 is analogous to that shown in FIG. 7. The amplifier 58 is equipped with dilferential input terminals 60 and 62 and is connected in a voltage follower configuration. This arrangement permits the power amplifier to act as its own limiter without causing loss of accuracy as long as the input terminals, particularly 60, do not draw appreciable current even with the amplifier saturated. This condition is easily met in practice. Capacitor 56 and resistor 24 perform the integration of the flux producing yoke voltage and comprise the high-frequency control loop. Resistors 38 and 54 provide current sensing and low-frequency feedback as before. Minor corrections for yoke and mounting structure distortions of the field are provided by networks 44, 46, and 48, 50 as in the arrangements of FIGS. 6 and 7.

Although a number of embodiments of the invention have been shown and described, it is apparent that many changes and modifications may be made by one skilled in the art without departing from the true scope and spirit of the invention.

What is claimed is:

1. A cathode ray tube deflection circuit comprising:

input means for receiving an input deflection signal;

a drive amplifier having an input connected to said input means and having an output;

a magnetic deflection yoke having an input connected to said output of said drive amplifier and having a resonant frequency; and

feedback means operatively associated with said deflection yoke for providing a feedback signal to said input of said amplifier representative of inductive current through said yoke, said inductive currents being at frequencies below and above said resonant frequency, said feedback means including means for providing a first signal representative of change of flux in said yoke, said means including a feedback connection from said input of said deflection yoke, and means for integrating said first means to provide said feedback signal.

2. The circuit defined =by claim 1, wherein said feedback means comprise an integrator being coupled between the input of said deflection yoke and said drive amplifier.

3. The circuit defined by claim 1, wherein said feedback means comprise:

first resistance means connected across said drive amplifier; and

first capacitance means connected between said input means and said input of said drive amplifier.

4. The circuit defined by claim 1, including means for preventing saturation of said integrating means.

5. The circuit defined by claim 3, further including second resistance means connected across said drive amplifier and across said capacitance means to provide low-frequency feedback.

6. The circuit defined by claim 5, further including at least one series combination of second resistance means and second capacitance means connected between said input of said deflection yoke and said input of said drive amplifier.

7. The circuit defined by claim 6, further including input signal limiting means connected between said input means and said input to said drive amplifier.

8. The circuit defined by claim 7 further including connection of resistance means in a manner to reduce effect of yoke resistance on low-frequency stability.

9. The circuit defined by claim 1 wherein integration of said first signal is accomplished by a resistance and capacitance means connected to a second input terminal of said drive amplifier thus providing the required feedback.

References Cited UNITED STATES PATENTS 2,890,381 6/1959 Sinnott 31527 2,913,625 11/ 1959 Finkelstein. 3,041,470 6/1962 Woodworth.

RODNEY D. BENNETT, JR., Primary Examiner J. G. BAXTER, Assistant Examiner 

